Servo writing devices for creating servo patterns with inherent track ID

ABSTRACT

The invention is directed to patterns of amplitude-based servo windows that eliminate the need for conventional track identification marks, and various servo writing devices and head configurations that facilitate creation of such servo patterns. The patterns of servo windows may also eliminate the need for conventional synchronization marks. In accordance with the invention, the patterns of servo windows, themselves, can distinguish one track from another track in one or more servo bands and inherently provide synchronization without the need for synchronization marks between servo frames. In other words, the same amplitude-based servo windows that define the centerlines of the tracks can be arranged to provide track identification information and possibly inherent synchronization information.

TECHNICAL FIELD

The invention relates to magnetic storage media and, more particularly,magnetic storage media recorded with amplitude-based servo patterns.

BACKGROUND

Data storage media are commonly used for storage and retrieval of data,and come in many forms, such as magnetic tape, magnetic disks, opticaltape, optical disks, holographic disks or cards, and the like. Magnetictape media remains economical for storing large amounts of data. Forexample, magnetic tape cartridges, or large spools of magnetic tape, areoften used to back up data in large computing centers. Magnetic tapecartridges also find application in the backup of data stored on smallercomputers such as desktop or notebook computers.

In magnetic media, data is typically stored as magnetic signals that aremagnetically recorded on the medium surface. The data stored on themedium is typically organized along “data tracks,” and transducer headsare positioned relative to the data tracks to write data to the tracksor read data from the tracks. A typical magnetic storage medium, such asmagnetic tape, typically includes several data tracks. Optical media,holographic media and other media formats can also make use of datatracks.

Servo patterns refer to signals or other recorded marks on the mediumthat are used for tracking purposes. In other words, servo patterns arerecorded on the medium to provide reference points relative to the datatracks. A servo controller interprets detected servo patterns andgenerates position error signals. The position error signals are used toadjust the lateral distance of the transducer head relative to the datatracks so that the transducer head is properly positioned along the datatracks for effective reading and/or writing of the data to the datatracks.

With some data storage media, such as magnetic tape, the servo patternsare stored in specialized tracks on the medium, called “servo tracks.”Servo tracks serve as references for the servo controller. Servo trackstypically hold no data except for information that is useful to theservo controller to identify positioning of a transducer head relativeto the surface of the medium. A plurality of servo tracks may be definedin a servo band. Some magnetic media include a plurality of servo bands,with data tracks being located between the servo bands.

The servo patterns recorded in the servo tracks may be sensed by one ormore servo heads. For example, servo heads may be dedicated heads thatread only servo patterns in the servo tracks. Alternatively, servo headsmay be integrated with a read/write head. In any case, once a particularservo track is located by the servo head, one or more data tracks can belocated on the medium according to the data track's known displacementfrom the servo track. The servo controller receives detected servosignals from the servo heads, and generates position error signals,which are used to adjust positioning of a read/write head relative tothe data tracks.

Servo patterns are referred to as pre-recorded when they are recordedduring the fabrication of the media. In other words, pre-recorded servopatterns are servo patterns recorded in the media prior to the mediabeing used for storage of data. These pre-recorded servo patterns allowthe media to achieve higher storage densities because the servo patternsenable positions on the media to be located with greater precision.Therefore, servo patterns allow for smaller amounts of media surface tobe used to store units of data.

Amplitude-based servo patterns refer to servo patterns in whichdetection of the servo signal amplitude enables identification of headpositioning relative to the medium. Amplitude-based servo patternstypically make use of amplitude-based servo windows which can berecorded or erased windows where a signal has been record or erased fromthe medium. As the head passes relative to the medium, signal amplitudesof detected servo signals can be used to determine whether the head ispositioned correctly relative to a track on the medium. Amplitude-basedservo patterns are commonly implemented in magnetic tape media, but mayalso be useful in other media.

SUMMARY

In general, the invention is directed to patterns of amplitude-basedservo windows that eliminate the need for conventional trackidentification marks, and various servo writing devices and headconfigurations that facilitate creation of such servo patterns. Thepatterns of servo windows may also eliminate the need for conventionalsynchronization marks. In accordance with the invention, the patterns ofservo windows, themselves, can distinguish one track from another trackin one or more servo bands and inherently provide synchronizationwithout the need for synchronization marks between servo frames. Inother words, the same amplitude-based servo windows that define thecenterlines of the tracks can be arranged to provide trackidentification information and possibly inherent synchronizationinformation.

In one embodiment, the invention provides a data storage mediumcomprising a first servo band including a first set of amplitude-basedservo windows, wherein the first set of amplitude-based servo windowsdefines a first set of servo tracks in the first servo band. The mediumfurther includes a second servo band including a second set ofamplitude-based servo windows, wherein the second set of amplitude-basedservo windows defines a second set of servo tracks in the second servoband, and wherein the second set amplitude-based servo windows isarranged differently than the first set of amplitude-based servowindows.

In another embodiment, the invention provides a system comprising a datastorage medium comprising a first servo band including a first set ofamplitude-based servo windows, wherein the first set of amplitude-basedservo windows defines a first set of servo tracks in the first servoband, and a second servo band including a second set amplitude-basedservo windows, wherein the second set of amplitude-based servo windowsdefine a second set of servo tracks in the second servo band, whereinthe second set amplitude-based servo windows is arranged differentlythan the first set of amplitude-based servo windows. The system alsoincludes a first head to pass relative to a given one of the servotracks in the first servo band, a second head to pass relative to acorresponding one of the tracks in the second servo band, and acontroller to determine whether the first and second heads arepositioned on-track based on amplitudes of signals detected by theheads.

In another embodiment, the invention provides a method comprisingrecording a first set of amplitude-based servo windows on a magneticmedium to define a first servo band including a first set of servotracks, and recording a second set of amplitude-based servo windows on amagnetic medium to define a second servo band including a second set ofservo tracks, wherein the second set of amplitude-based servo windows isarranged differently than the first set of amplitude-based servowindows.

In another embodiment, the invention provides a method comprisingdetecting a first servo signal along a servo track of a first servo bandbased on amplitude-based servo windows in the first servo band,detecting a second servo signal along a corresponding servo track of asecond servo band based on amplitude-based servo windows in the secondservo band, and identifying the servo track based on the first andsecond servo signals.

In another embodiment, the invention provides a data storage mediumcomprising a servo band including a set of amplitude-based servowindows. The servo band includes a first servo track defined by a firstsubset of the amplitude-based servo windows positioned above and below afirst centerline, a second servo track adjacent the first servo track,defined by a second subset of the amplitude-based servo windowspositioned above and below a second centerline, and a third servo trackadjacent the second servo track, defined by a third subset of theamplitude-based servo windows positioned above and below a thirdcenterline. The amplitude-based servo windows are arranged such thatoutput signals associated with the first and third servo tracks areunique relative to one another.

In another embodiment, the invention provides a data storage mediumcomprising a servo band including a set of amplitude-based servowindows, wherein set of the amplitude-based servo windows define a setof servo tracks in the servo band, wherein each track is defined by acenterline having one or more of the amplitude-based servo windows abovethe centerline and one or more of the amplitude-based servo windowsbelow the centerline, and the set of amplitude-based servo windows isarranged in a stepped configuration in a direction perpendicular to theservo tracks.

In another embodiment, the invention provides a system comprising a datastorage medium comprising a servo band including a set ofamplitude-based servo windows, the servo band further including a firstservo track defined by a first subset of the amplitude-based servowindows positioned above and below a first centerline, a second servotrack adjacent the first servo track, defined by a second subset of theamplitude-based servo windows positioned above and below a secondcenterline, and a third servo track adjacent the second servo track,defined by a third subset of the amplitude-based servo windowspositioned above and below a third centerline, wherein theamplitude-based servo windows are arranged such that output signalsassociated with the first and third servo tracks are unique relative toone another. The system also includes a head to pass relative to themedium along one of the tracks, and a controller to identify which ofthe tracks the head is passing based on signals detected by the head.

In another embodiment, the invention provides a method comprisingrecording a servo band on a magnetic medium, the servo band including aset of amplitude-based servo windows, the servo band further including afirst servo track defined by a first subset of the amplitude-based servowindows positioned above and below a first centerline, a second servotrack defined by a second subset of the amplitude-based servo windowspositioned above and below a second centerline, and a third servo trackdefined by a third subset of the amplitude-based servo windowspositioned above and below a third centerline, wherein theamplitude-based servo windows are arranged such that output signalsassociated with the first, second, and third servo tracks are uniquerelative to one another.

In another embodiment, the invention provides a method comprisingrecording a servo band on a magnetic medium, the servo band including aset of amplitude-based servo windows, wherein the set of theamplitude-based servo windows define a set of servo tracks in the servoband, and wherein each track is defined by a centerline having one ormore of the amplitude-based servo windows above the centerline and oneor more of the amplitude-based servo windows below the centerline, andthe set of amplitude-based servo windows is arranged in a steppedconfiguration in a direction perpendicular to the servo tracks.

In another embodiment, the invention provides a data storage mediumcomprising a first servo track defining a first centerline, a firstamplitude-based servo window above the first centerline, and a secondamplitude-based servo window below the first centerline, wherein a widthof the first amplitude-based servo window is different than a width ofthe second amplitude-based servo window. The medium also includes asecond servo track defining a second centerline, wherein the secondamplitude-based servo window is above the second centerline, a thirdamplitude-based servo window below the second centerline wherein a widthof the second amplitude-based servo window is different than a width ofthe third amplitude-based servo window, a third servo track defining athird centerline, wherein the third amplitude-based servo window isabove the third centerline, and a fourth amplitude-based servo windowbelow the third centerline wherein a width of the forth amplitude-basedservo window is different than a width of the third amplitude-basedservo window, and wherein either the first and third amplitude-basedservo windows or the second and fourth amplitude-based servo windowsdefine different widths respectively.

In another embodiment, the invention provides a servo writing devicecomprising a first electromagnetic element to generate a magnetic fieldto record signal on a magnetic medium, a second electromagnetic elementto selectively erase the signal, and a layer formed over the secondelectromagnetic element to define a set of gaps. The servo writingdevice also includes a controller to control the second electromagneticelement such that a magnetic field pattern is generated from the set ofgaps for defined periods of time as the magnetic medium passes relativeto the servo writing device such that amplitude-based servo windows arecreated on the medium by the set of gaps.

In another embodiment, the invention provides a servo writing devicecomprising an electromagnetic element to generate a magnetic field, anda layer formed over the electromagnetic element to define a first set ofgaps that define a first magnetic field pattern corresponding to a firstservo band on a magnetic medium and a second set of gaps that define asecond magnetic field pattern corresponding to a second servo band onthe magnetic medium.

In another embodiment, the invention provides servo writing devicecomprising a first electromagnetic element to generate a magnetic fieldto record signal on a magnetic medium, a second electromagnetic elementto selectively erase the signal, and a layer formed over the secondelectromagnetic element to define a first set of gaps that define afirst pattern corresponding to a first servo band on a magnetic mediumand a second set of gaps that define a second pattern corresponding to asecond servo band on the magnetic medium.

In another embodiment, the invention provides a servo writing devicecomprising an electromagnetic element to generate a magnetic field, anda layer formed over the electromagnetic element to define a set of gapsthat define a magnetic field pattern corresponding to a servo band on amagnetic medium, wherein the set of gaps is arranged in an steppedconfiguration in which centerlines of tracks for the servo band aredefined between each step in the stepped configuration of the gaps.

In another embodiment, the invention provides a servo writing devicecomprising an electromagnetic element to generate a magnetic field, anda layer formed over the electromagnetic element to define a set of gapsthat define a magnetic field pattern corresponding to a servo band on amagnetic medium. The set of gaps is arranged to define a first subset ofgaps positioned above and below a location corresponding to a firstcenterline of the servo band and corresponding to a first servo track, asecond subset of gaps positioned above and below a locationcorresponding to a second centerline and corresponding to a second servotrack of the servo band, and a third subset of gaps positioned above andbelow a location corresponding to a third centerline of the servo bandand corresponding to a third servo track of the servo band, wherein thefirst subset of gaps, the second subset of gaps, and the third subset ofgaps are unique relative to one another.

Various aspects of the invention can provide a number of advantages. Ingeneral, amplitude-based servo patterns can facilitate the ability topinpoint locations on media surfaces with greater accuracy. Therefore,the described servo patterns can allow for smaller amounts of mediasurface to be used to store units of data. More particularly, the servopatterns described herein can eliminate the need for conventional trackidentification marks, e.g., which conventionally cross the centerlines.The servo patterns may also eliminate the need for conventionalsynchronization marks which also cross the centerlines. Eliminating theneed for track identification marks and/or synchronization marks cansimplify the process of recording the amplitude-base servo patterns onthe medium, possibly improving media quality and/or reducing mediamanufacturing costs. Moreover, servo patterns making use of differentlysized servo windows can simplify the pattern relative to conventionalamplitude-based patterns by eliminating the need for additional marksconventionally used for synchronization in the servo detection process.Instead, widths of the differently sized servo windows can be measuredin order to provide self-synchronization.

Additional details of these and other embodiments are set forth in theaccompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a depiction of a servo band portion of a prior art mediumrecorded with a conventional amplitude-based servo pattern.

FIG. 2 is another depiction of a servo band portion of a prior artmedium recorded with a conventional amplitude-based servo pattern.

FIG. 3 is a depiction of a prior art medium recorded with conventionalamplitude-based servo patterns.

FIG. 4 is a depiction of a medium recorded with amplitude-based servopatterns according to an embodiment of the invention.

FIGS. 5A-5C is a depiction of the medium of FIG. 4 with servo read headspassing over the medium at various locations.

FIGS. 6-8 illustrate exemplary readout signals associated with servoread heads passing over a medium as shown in FIGS. 5A-5C respectively.

FIG. 9 is a depiction of a medium recorded with amplitude-based servopatterns according to an embodiment of the invention.

FIG. 10 is a depiction of a medium with servo read heads passing overthe medium according to an embodiment of the invention.

FIGS. 11 and 12 illustrate exemplary readout signals associated withservo read heads passing over a medium as shown in FIG. 10.

FIG. 13 is a depiction of a medium recorded with amplitude-based servopatterns according to an embodiment of the invention.

FIG. 14 is a depiction of a medium recorded with an amplitude-basedservo pattern according to another embodiment of the invention.

FIG. 15 is a depiction of a medium recorded with an amplitude-basedservo pattern according to an embodiment of the invention.

FIGS. 16, 18 and 20 illustrate a magnetic head relative to a mediumrecorded with a servo pattern according to an embodiment of theinvention.

FIGS. 17, 19 and 21 illustrate the corresponding readout signalsrespectively associated with the magnetic head passing relative to themedium as illustrated in FIGS. 16, 18 and 20.

FIGS. 22 and 23 are block diagrams illustrating exemplary data storagesystems according to embodiments of the invention.

FIG. 24 illustrates a readout signal and a corresponding envelope signalthat may be generated when a servo read head passes relative to a servotrack substantially along a centerline of a servo track.

FIG. 25 illustrates pulses that can be generated based on the envelopesignal illustrated in FIG. 24.

FIG. 26 illustrates a readout signal and a corresponding envelope signalthat may be generated when servo head passes relative to a servo trackslightly below a centerline of a servo track.

FIG. 27 illustrates pulses that can be generated based on the envelopesignal illustrated in FIG. 26.

FIG. 28 illustrates a readout signal and a corresponding envelope signalthat may be generated when a servo head passes relative to a servo trackslightly above a centerline of a servo track.

FIG. 29 illustrates pulses that can be generated based on the envelopesignal illustrated in FIG. 28.

FIG. 30 is a block diagram illustrating an exemplary servo writingdevice for pre-recording servo patterns on a data storage medium asdescribed herein.

FIG. 31 is a cross-sectional conceptual view of an exemplary servo writehead and a servo erase head that form a servo writing device.

FIG. 32 is a top view of an exemplary servo writing device comprising aservo write head and a servo erase head separated by a magnetic shield.

FIG. 33 is a top view of another servo erase head according to anembodiment of the invention.

FIG. 34 is a top view of another servo erase head according to anembodiment of the invention.

FIG. 35 is a top view of another servo erase head according to anembodiment of the invention.

FIG. 36 is a top view of another servo erase head according to anembodiment of the invention.

FIG. 37 illustrates a servo erase head similar to that of FIG. 35passing over a medium to erase servo windows in the medium.

FIG. 38 is a depiction of a medium including a plurality of servo bandsand data bands between the servo bands.

DETAILED DESCRIPTION

FIG. 1 is a depiction of a servo band portion of a prior art medium 10recorded with a conventional amplitude-based servo pattern. Theconventional servo pattern illustrated in FIG. 1 includes a number ofservo windows 12A-12F. Servo windows 12 may comprise areas where apreviously recorded magnetic signal 17 has been erased from medium 10.Medium 10 includes three servo tracks 14A, 14B, 14C that definetracklines 15A, 15B, 15C. As a head moves over medium 10 relative to oneof servo tracks 14, the strength of the magnetic signal detected by thehead can identify the location of the head relative to a given one oftracklines 15.

For example, as a servo head moves partially over servo window 12A alongtrackline 15A, the detected signal amplitude should reduce by 50 percentif the head is precisely on-track. The detected signal is 100 percentwhen the head is not passing over a servo window, but reduces when thehead passes partially over a servo window because the part of the headpassing over the servo window is not exposed to a signal. If thedetected signal amplitude falls by an amount greater or less than 50percent as the head partially over servo window 12A along trackline 15A,then the head can be moved to better position the head over trackline15A. In this manner, tracklines 15 of servo tracks 14 can be located.Corresponding data tracks (not shown) are located at defineddisplacements from tracklines 15 of servo tracks 14.

FIG. 2 is another depiction of a servo band portion of a prior artmedium 20 recorded with a conventional amplitude-based servo pattern.The conventional servo pattern in FIG. 2 includes two servo frames 22Aand 22B. Each frame includes five servo tracks 24A, 24B, 24C, 24D and24E. These five servo tracks 24 collectively define a servo band 26. Ingeneral, a servo band is defined as a collection of a plurality of servotracks. Thus, a servo band could include any number of servo tracks.Each of servo tracks 24 may reside a known distance from a correspondingdata track (not shown).

The servo pattern can be written by passing the medium under gaps of aservo write head. A relatively wide gap in the servo head can be used torecord a magnetic signal 27 having a first frequency on the surface ofmedium 20. Moreover, a magnetic signal having a second frequency maydefine transition regions 28A and 28B between the individual frames. Torecord transition regions 28A and 28B, the frequency of the writtensignal is changed for a short period of time while the tape passes underthe wide gap in the servo head. The transition regions 28 serve assynchronization marks in the prior art servo detection scheme.

A servo write head (or a separate erase head) having a relatively smallwrite gap track width in the direction transverse to the servo trackdirection can be used to create erased servo windows 29. For example,erased servo windows 29A-29L (collectively erased servo windows 29) mayform a checkerboard-like configuration that enables a read head topinpoint track locations. In accordance with the prior art, the erasedservo windows 29 respectively positioned above and below centerline 25have a common width.

In operation, as medium 20 passes by a read head (not shown) positionedover a first track (indicated by numeral 24A), the position of erasedservo windows 29A and 29B or 29G and 29H relative to magnetic signal 27,can accurately define the track location of the head. Similarly, thetrack locations of tracks 2-5 (indicated by numerals 24B-24Erespectively) can be defined by the various erased servo windows 29relative to magnetic pattern 27. Detection of transition regions 28provides a synchronization mechanism so that when signal amplitudeindicates head positioning that is off-track, the servo controller candetermine whether to cause movement of the magnetic head laterally up ordown in order to remedy the off-track head positioning. With commonsized servo windows 29, the head controller could become out of sync iftransition regions 28 or another type of synchronization mechanism isnot recorded on medium 20 between servo frames 22. Transition regions 28typically cross one or more centerlines 25.

Medium 20 also includes track identification marks 23A, 23B. Trackidentification marks 23 allow a servo controller to distinguish track24A from tracks 24C and 24E. Without track identification marks 23 onmedium 20, the detected signals associated with tracks 24A, 24C and 24Eare generally indistinguishable. For this reason, medium 20 includestrack identification marks 23 to distinguish track 24A from tracks 24Cand 24E. For example, track identification marks 23 may comprise amagnetic signal having a different discernable frequency than signal 17.Track identification marks 23 are conventionally shaped differently thanservo windows 29. Also, unlike servo windows 29, track identificationmarks 23 are not positioned or used for amplitude-based servopositioning. For example, track identification marks 23 typically crossone or more centerline 25. Adjacent servo bands may include trackidentification marks similar to marks 23, but positioned differentlywithin the given band, so that tracks 24C, 24D and 24E can beidentified. The discussion of FIG. 3 provides additional details ofprior art use of conventional track identification marks.

FIG. 3 is a prior art depiction of medium 30 relative to a servo readdevice 35 including three magnetic heads 31A, 31B, 31C. In particular,servo read device 35 is illustrated in three exemplary location relativeto medium 30. Medium 30 includes servo bands 32A, 32B and 32C. Datatracks 36 are positioned relative to servo bands 32. Each servo band 32defines a plurality of servo tracks. Each servo band 32 includessynchronization marks 38 (only synchronization marks 38 of servo band32A are labeled in FIG. 3 although every servo band 32 includes similarsynchronization marks). Moreover, each servo band 32 includes trackidentification marks 33A-33C. Track identification marks 33A are locatedto cross the centerline of a first track of servo band 32A. However,track identification marks 33B are located to cross the centerline of athird track of servo band 32B, and track identification marks 33C arelocated to cross the centerline of a fifth of servo band 32C.

Servo device 35 including servo heads 31A, 31B and 31C is illustrated inthree different locations relative to medium 30, i.e., locations 37A,37B and 37C. In particular, servo heads 31A, 31B and 31C are illustratedalong centerlines associated with a first track, a third track, and afifth track of servo bands 32. When servo heads 31 are positioned alongthe centerline of the first track of servo bands 32 (as shown at 37A) orthe second track, servo head 31A detects track identification marks 33A.When servo heads 31 are positioned along the centerline of the thirdtrack (as shown at 37B) or the fourth track, servo head 31B detectstrack identification marks 33B. When servo heads 31 are positioned alongthe centerline of the fifth track of servo bands 32 (as shown at 37C) orthe fourth track, servo head 31C detects track identification marks 33C.In general, track identification marks 33 of different servo bands 32are positioned to cross different centerlines. Accordingly,identification marks 33 can be detected by the different servo heads 31of servo device 35 to distinguish the tracks of servo bands 32.

The creation of conventional synchronization marks and trackidentification marks such as track identification marks 33A, 33B and33C, however, can be difficult. Conventional synchronization marks andtrack identification marks are typically created by recording magneticsignals at different frequencies than other signals recorded in servobands 32. Modulating the frequency during magnetic recording isdifficult, particularly when precise positioning of the servosynchronization marks or track identification marks is needed. Ingeneral, creating conventional synchronization marks and trackidentification marks adds complexity and cost to the fabrication ofmagnetic media.

The invention is directed to patterns of amplitude-based servo windowsthat eliminate the need for conventional track identification marks. Inaddition, the patterns of servo windows may eliminate the need forconventional synchronization marks. In accordance with the invention,the patterns of servo windows, themselves, can distinguish one trackfrom another track in one or more servo bands. In other words, the sameamplitude-based servo windows that define the centerlines of the trackscan be arranged to provide track identification information and possiblyinherent synchronization information. In accordance with the invention,track identification marks that cross one or more of the centerlines canbe completely eliminated. Also, synchronization marks that cross one ormore of the centerlines can also be eliminated. Again, the eliminationof conventional track identification marks and conventionalsynchronization marks can simplify media fabrication.

In some embodiments, different patterns of servo windows are defined indifferent bands of the medium such that a collective servo output signalassociated with two similar tracks of two different servo bands isunique. In other words, a medium may include a first servo bandincluding a first set of amplitude-based servo windows and a secondservo band including a second set of amplitude-based servo windows,wherein the second set amplitude-based servo windows are arrangeddifferently than the first set of amplitude-based servo windows. Inother words, the first and second sets of amplitude-based servo windowsdefine different servo patterns. In that case, the collective outputassociated with corresponding servo tracks of the two servo bands can beunique for each set of corresponding tracks of the servo bands.

In other embodiments, patterns of servo windows are defined such thatevery track of a given servo band defines a unique signal relative tothe other tracks of that band. In other words, each track of a givenservo band may define a unique output signal that can be interpreted tofacilitate on-track positioning of a head and also identification of thetrack. In these ways, the need for conventional track identificationmarks and possibly synchronization marks can be eliminated. Accordingly,the invention can simplify media fabrication by eliminating the need tocreate such conventional track identification marks and/orsynchronization marks.

Also described are various techniques and servo head configurationsuseful for creating the servo patterns described herein. In particular,servo writing devices formed with patterns of servo gaps are describedthat can facilitate relatively simple creation of complex patterns ofservo windows, which inherently include synchronization and trackidentification information. Servo writing devices for simultaneouslycreating different patterns of servo windows in different servo bandsare described. Also, servo writing devices for creating specificpatterns in an individual servo band are described.

In general, the servo controller always knows what track it is seeking.The controller also knows how the position error signal will behave onthe track that it is seeking, e.g., whether upward motion of the headcauses greater or lesser signal during the time the head is near a givenservo window. In other words, the polarity of the position error signalcan be defined and known by the controller for any given track number.If an open loop servo system positions the head in the vicinity of theproper track, i.e., within one track pitch, when the loop is closed, theservo controller will cause the head to fall toward the desired andproper window edge.

Put another way, when the servo controller knows the expected behaviorof a given track, the opposing edge of servo windows (corresponding toadjacent tracks) is not a stable closed loop position. Having two servowindows per track (one window on either side of track centerline)complicates track interpretation by the controller. If the windows arethe same size and uniformly spaced (as a checkerboard), all the trackslook the same to the controller. Another element must therefore be addedto provide the required differentiation and synchronization. This isprovided in the prior art as a frequency shift in the un-windowedportion of the servo band. One aspect of this invention, however,provides this needed synchronization by altering the widths of servowindows for one or more of the tracks.

FIG. 4 is a depiction of medium 40 according to an embodiment of theinvention. Medium 40 includes servo bands 41A-41C and data bands 42A,42B. Data bands 42A, 42B are respectively positioned between thedifferent servo bands 41. Each of servo bands 41 define a plurality ofservo tracks. In particular, centerlines 43A, 43B and 43C correspond tothe servo tracks of respective servo bands 41. Sets of amplitude-basedservo windows 45 are arranged with respect to centerlines 43, e.g., withindividual windows typically being adjacent one or more centerlines 43.The amplitude-based servo windows in sets 45 are generally notpositioned to cross any of centerlines 43. As illustrated, the sets ofservo windows 45 repeat to define successive servo frames along thelength of medium 40.

Medium 40 includes a first servo band 41A including a first set ofamplitude-based servo windows 45A. The first set of amplitude-basedservo windows 45A defines a first set of servo tracks in first servoband 41A. In addition, medium 40 includes a second servo band 41Bincluding a second set of amplitude-based servo windows 45B. The secondset of amplitude-based servo windows 45B define a second set of servotracks in second servo band 41B. Amplitude-based servo windows 45 maycomprise areas where a recorded magnetic signal has been erased frommedium 40, or alternatively the servo windows 45 may be magneticallyrecorded, rather than erased. In accordance with the invention, thesecond set of amplitude-based servo windows 45B are arranged differentlythan the first set of amplitude-based servo windows 45A. Again,amplitude-based servo windows 45 are generally not positioned to crossany of centerlines 43, unlike conventional identification marks orsynchronization marks.

In the example of FIG. 4, first set of amplitude-based servo windows 45Ais arranged in a checkerboard-like configuration, whereas second set ofamplitude-based servo windows 45B is arranged in a steppedconfiguration. First set of amplitude-based servo windows 45A may bearranged similarly to conventional amplitude-based servo patterns.However, second set of amplitude-based servo windows 45B is arrangeddifferently, e.g., in a stepped configuration in a directionperpendicular to the servo tracks. The stepped configuration of secondset of amplitude-based servo windows 45B may define downward steps (asillustrated), or upward steps. The upward or downward steps generallyrefer to steps in a direction transverse to the length of the medium.

In any case, with first and second sets of amplitude-based servo windows45A, 45B arranged differently, the collective output signals associatedwith two different heads moving over first and second servo bands 41A,41B may be unique for any given set of tracks. Accordingly, the need forconventional identification marks can be eliminated. Instead, patternsof servo windows 45A, 45B can inherently include track identificationinformation that can be detected by servo read heads and identified by acorresponding servo controller. In particular, the differentarrangements, e.g., checkerboard versus stepped, serve to uniquelyidentify a first track associated with the different arrangements from asecond track associated with the different arrangements.

FIGS. 5A-5C are depictions of a servo device 52 passing relative tomedium 40. In the example of FIGS. 5A-5C, servo device 52 includes threeservo heads 53A, 53B, 53C positioned in servo device 52 tosimultaneously track respective servo tracks of servo bands 41A, 41B and41C. For example, servo heads 53A, 53B, 53C may comprise magnetictransducer heads that detect magnetic signals on the surface of medium40. FIG. 5A illustrates servo heads 53A, 53B, 53C passing alongcenterlines 56A, 56B and 56C, which correspond to first servo tracks ofservo bands 41. FIG. 5B illustrates servo heads 53A, 53B, 53C passingalong centerlines 57A, 57B and 57C, which correspond to second servotracks of servo bands 41. FIG. 5C illustrates servo heads 53A, 53B, 53Cpassing along centerlines 58A, 58B and 58C, which correspond to thirdservo tracks of servo bands 41.

FIG. 6 illustrates exemplary output signals corresponding to FIG. 5A. Inparticular, FIG. 6 illustrates output signals 61, 62 associated withfirst servo head 53A and second servo head 53B, as heads 53A and 53Bpass over region 49 of medium 40 along centerlines 56A and 56B. Ingeneral, variations in the amplitudes (A) of signals 61, 62 can be usedto identify whether heads 53 are on-track. Signals 61 and 62 generallyindicate on-track positioning because the amplitudes of signals 61 and62 fall by 50 percent as heads 53A and 53B pass partially over servowindows in servo bands 41. If signals 61 and 62 fall by greater or lessthan 50 percent (at locations 63, 64), then position error signalsgenerated by a servo controller should remedy the off-track positioning.In particular, the position error signals cause movement of heads 53 topositions that ensure that output signals appear like signals 61 and 62,which indicate on-track positioning with respect to centerlines 56A and56B.

FIG. 7 illustrates exemplary output signals corresponding to FIG. 5B. Inparticular, FIG. 7 illustrates output signals 71, 72 associated withfirst servo head 53A and second servo head 53B, as heads 53A and 53Bpass over region 49 of medium 40 along centerlines 57A and 57B. Ifsignals 71 and 72 fall by greater or less than 50 percent (at locations73, 74), then position error signals generated by a servo controllershould remedy the off-rack positioning. In that case, the position errorsignals cause movement of heads 53 to positions that ensure that outputsignals appear like signals 71 and 72, which indicate on-trackpositioning with respect to centerlines 57A and 57B.

FIG. 8 illustrates output signals 81, 82 associated with first servohead 53A and second servo head 53B, as heads 53A and 53B pass overregion 49 of medium 40 along centerlines 58A and 58B. If signals 81 and82 fall by greater or less than 50 percent (at locations 83, 84), thenposition error signals generated by a servo controller should remedy theoff-track positioning. Again, the position error signals cause movementof heads 53 to ensure that output signals appear like signals 81 and 82,which indicate on-track positioning with respect to centerlines 58A and58B.

First and second sets of amplitude-based servo windows 45A, 45B arearranged differently, such that output signals 61 and 62 arecollectively unique relative to output signals 71, 72 and output signals81, 82. Similarly, output signals 71, 72 are collectively unique, andoutput signals 81, 82 are collectively unique. For example, offsets O₁,O₂ and O₃ are different for the different pairs of output signals, i.e.,output signals 61, 62, output signals 71, 72 and output signals 81, 82.A servo controller can identify offsets O₁, O₂ or O₃ in order toidentify the given servo track currently being read by servo device 52.Accordingly, the need for conventional track identification marks can beeliminated.

FIG. 9 is a depiction of medium 90 according to an embodiment of theinvention. Medium 90 includes servo bands 91A-91C and data bands 92A,92B between the servo bands. Each of servo bands 91 define a pluralityof servo tracks. In particular, centerlines 93A, 93B and 93C correspondto the servo tracks of respective servo bands 91. The amplitude-basedservo windows in sets 95 are arranged with respect to centerlines 93,e.g., with individual windows typically being adjacent one or morecenterlines 93. The amplitude-based servo windows in sets 95 aregenerally not positioned to cross any of centerlines 93. As illustrated,the sets of servo windows 95 repeat to define successive servo framesalong the length of medium 90.

Medium 90 includes a first servo band 91A including a first setamplitude-based servo windows 95A. The first set of amplitude-basedservo windows 95A defines a first set of servo tracks in first servoband 91A. In addition, medium 90 includes a second servo band 91Bincluding a second set amplitude-based servo windows 95B. The second setof amplitude-based servo windows 95B defines a second set of servotracks in second servo band 91B. Amplitude-based servo windows 95 maycomprise areas where a recorded magnetic signal has been erased frommedium 90. Alternatively, servo windows 95 may be magnetically recordedwith a magnetic signal, rather than erased. In accordance with theinvention, the second set of amplitude-based servo windows 95B arearranged differently than the first set of amplitude-based servo windows95A.

In this example, first set of amplitude-based servo windows 95A isarranged in a upward stepped configuration relative to a traversedirection of medium 90, whereas second set of amplitude-based servowindows 95B are arranged in a downward stepped configuration. In anycase, with first and second sets of amplitude-based servo windows 95A,95B arranged differently, the collective output signals associated withtwo different heads moving over first and second servo bands 91A, 91Bcan be unique for any given set of tracks. Accordingly, the need forconventional identification marks can be eliminated. Instead, patternsof servo windows 95A, 95B can inherently include track identificationinformation that can be identified by servo read heads.

In other embodiments, any number of servo bands may be formed on amedium. The arrangement of amplitude-based servo windows in thedifferent bands may alternate. In other words, every other servo bandmay have a similar arrangement of servo windows, but adjacent servobands may define different arrangements, in order to facilitate trackidentification as described herein. In still other embodiments, eachservo band may define a unique pattern of servo windows relative to allother servo bands. In any case, arrangement of amplitude-based servowindows as described herein can encode more information thanconventional patterns of servo windows. For example, trackidentification information and possibly synchronization information maybe encoded in the patterns of amplitude-based servo windows. Differentpatterns in different servo bands, in particular, is a useful way toencode track identification information in the arrangements ofamplitude-based servo windows.

FIG. 10 illustrates a pair of servo bands 101, 102, similar to servobands 41A and 41B (FIG. 4). Servo band 101 may include an arrangement ofamplitude-based servo windows that are arranged similarly toconventional checkerboard-like configurations. Servo band 102 differsfrom servo band 101 such that the longitudinal position of eachsuccessive pair of amplitude-based servo windows is offset from eachother. In other words, longitudinal centerline 104 of servo band 101 iscoincident, whereas longitudinal centerlines 105A-105D of successiveamplitude-based servo windows in band 102 are not coincident with eachother. Instead, longitudinal centerlines 105A-105D differ by a distancethat can be discerned by a servo controller that decodes detectedsignals.

Servo device 108 contains at least two independent servo read heads109A, 109B, one of which is positioned proximate to servo band 101, andthe other to servo band 102. If servo read head 109A is positioned overcenterline 106A associated with a first track of servo band 101, thenservo read head 109B will be similarly positioned over centerline 107Aassociated with a first track of servo band 102. If servo head read 109Ais positioned over centerline 106B associated with a second track ofservo band 101, then servo head read 109B will be similarly positionedover centerline 107B associated with a second track of servo band 102.This way, heads 109, can detect the relative distance betweenamplitude-based servo windows in bands 101 and 102, and a servocontroller can interpret the distance into a transverse or cross-tapeposition of servo device 108. Accordingly, the need for additional trackidentification marks can be eliminated. In FIG. 10, however, the amountof stagger of the windows in band 102 would be insufficient to allow acontroller to differentiate longitudinal centerline 105A longitudinalcenterline 105D. For this reason, additional stagger would be preferred,e.g., without overlap between the staggered windows.

FIG. 11 shows output waveforms 115, 116 respectively detected by servoheads 109A and 109B when head 109A is tracking on centerline 106B, andhead 109B is tracking on centerline 107B. The relative distance betweenservo window pairs 11B and 111C, and 112B and 112C can be discerned bydetecting timing offset 117. Similarly, FIG. 12 shows output waveformsfrom servo heads 109A and 109B when head 109A is tracking on centerline106C and head 109B is tracking on centerline 107C. Time offsets 117 and118 are different, and therefore can be used to uniquely define thetrack positions of heads 101, 102.

FIG. 13 is a depiction of medium 130 according to an embodiment of theinvention. Medium 130 includes servo bands 131A-131C and data bands132A, 132B between servo bands 131. Each of servo bands 131 defines aplurality of servo tracks. In particular, centerlines 133A, 133B and133C correspond to the servo tracks of respective servo bands 131. Theamplitude-based servo windows in sets 135 are arranged with respect tocenterlines 133, e.g., with individual windows typically being adjacentone or more centerlines 133. The amplitude-based servo windows in sets135 are generally not positioned to cross any of centerlines 133. Asillustrated, the sets of servo windows 135 repeat to define successiveservo frames along the length of medium 130.

In particular, medium 130 includes a first servo band 131A including afirst set amplitude-based servo windows 135A. The first set ofamplitude-based servo windows 135A defines a first set of servo tracksin first servo band 131A. In addition, medium 130 includes a secondservo band 131B including a second set amplitude-based servo windows135B, wherein the second set of amplitude-based servo windows 135Bdefines a second set of servo tracks in second servo band 131B. Inaccordance with the invention, the second set of amplitude-based servowindows 135B is arranged differently than the first set ofamplitude-based servo windows 135A.

In this example, first set of amplitude-based servo windows 135A isfurther defined to facilitate synchronization. In particular, two ormore of the different windows of the first set of amplitude-based servowindows 135A defines widths that are different relative to one another.For example, with respect to centerline 133, the amplitude-based servowindow 139A adjacent and above centerline 133 defines a width that isdifferent than the amplitude-based servo window 139B adjacent and belowcenterline 133. A servo controller can decode and distinguish signalsassociated with amplitude-based servo window 139A from signalsassociated with amplitude-based servo window 139B. Such features allowthe servo decoding system to synchronize to the servo pattern withoutthe need for conventional synchronization marks, such as marks 37 (FIG.3).

Additional details of servo patterns similar to those illustrated inservo band 131A, that allow for self-synchronization without the needfor conventional synchronization marks, are outlined in co-pending andcommonly assigned U.S. patent application Ser. No. 10/464,394, filedJun. 17, 2003 for Molsted and Yip, entitled AMPLITUDE-BASED SERVOPATTERNS FOR MAGNETIC MEDIA, and bearing attorney docket number10379US01, said application being hereby incorporated by referenceherein in its entirety.

FIG. 14 is a depiction of a servo band portion of a medium 140 recordedwith a amplitude-based servo pattern according to another embodiment ofthe invention. The servo pattern illustrated in FIG. 14 includes a servoband 146 defining a plurality of servo tracks 144A-144E. A set of servowindows 142A-142F define servo tracks 144A-144E. As illustrated, the setof servo windows 142 repeats for successive servo frames 143A-143B.Servo windows 142 may comprise areas where a magnetic signal 147 hasbeen erased from medium 140. Alternatively, servo windows 142 may bemagnetically written, rather than erased.

Servo band 146 includes a first servo track 144A defined by a firstsubset of the amplitude-based servo windows 142A, 142B positioned aboveand below a first centerline 145A, e.g., with each of servo windows142A, 142B typically being adjacent centerline 145A. Servo band 146 alsoincludes a second servo track 144B adjacent first servo track 144A anddefined by a second subset of the amplitude-based servo windows 142B,142C positioned above and below a second centerline 145B. In addition,servo band 146 includes a third servo track 144C adjacent second servotrack 144B and defined by a third subset of the amplitude-based servowindows 142C, 142D positioned above and below a third centerline 145C.In accordance with the invention, amplitude-based servo windows 142 arearranged such that output signals associated with the first servo track144A and third servo track 144C are unique relative to one another.

In particular, in medium 140, a distance between the amplitude-basedservo windows in the first subset 142A, 142B that are above and belowfirst centerline 145A is different than a distance between theamplitude-based servo windows in the third subset 142C, 142D that areabove and below the third centerline 144C. In other words, offset (O₁)between the trailing edge of servo window 142A and the leading edge ofservo window 142B is different than offset (O₂) between the trailingedge of servo window 142C and the leading edge of servo window 142D. Theoffsets (O₁) or (O₂) can be discerned by a servo controller andinterpreted to identify track location relative to medium 140. Trackidentification between the first track 144A and the second track 144Bmay be discerned by identification of the fact that servo window 142B isbelow centerline 145A for first track 144A and above centerline 145B forsecond track 144B.

In embodiments like FIG. 14, where a single servo band includes inherenttrack identification information, the servo controller would distinguishbetween tracks which share a common servo window. In particular, for apattern like that of FIG. 14, the controller would differentiate betweentracks 144A and 144B, and would also differentiate between tracks 144Cand 144D. This can be accomplished by the controller testing the sign ofthe position error signal as the system acquires a track. In this case,the track identification feature is inherent in the servo window pairs.As the controller locks onto the track, if movement of the servo headupward increases the amplitude from servo sample, then the head must beon track 144B because increased output from the first servo sample isassociated with head movement off of servo window 142C. Similarly, ifthe amplitude decreases, than the head must be on 144A.

FIG. 15 is a depiction of a servo band portion of a medium 150 recordedwith a amplitude-based servo pattern according to another embodiment ofthe invention. The servo pattern illustrated in FIG. 15 includes a servoband 156 defining a plurality of servo tracks 154A-154E. A set of servowindows servo windows 152A-152F defines servo tracks 154A-154E. Asillustrated, the set of servo windows 152 repeat for successive servoframes 153A-153B. Servo windows 152 may comprise areas where a magneticsignal 157 has been erased from medium 150. Alternatively, servo windows152 may be magnetically recorded with a magnetic signal, rather thanerased.

Servo band 156 includes a first servo track 154A defined by a firstsubset of the amplitude-based servo windows 152A, 152B positioned aboveand below a first centerline 155A, e.g., with each of servo windows152A, 152B typically being adjacent centerline 155A. Servo band 156 alsoincludes a second servo track 154B adjacent first servo track 154A anddefined by a second subset of the amplitude-based servo windows 152B,152C positioned above and below a second centerline 155B. In addition,servo band 156 includes a third servo track 154C adjacent second servotrack 154B and defined by a third subset of the amplitude-based servowindows 152C, 152D positioned above and below a third centerline 155C.In accordance with the invention, amplitude-based servo windows 152 arearranged such that output signals associated with the first servo track154A and third servo track 154C are unique relative to one another.

In particular, in medium 150, a width of at least one of theamplitude-based servo windows in the first subset 152A, 152B isdifferent than a width of at least one of the amplitude-based servowindows in the third subset 152C, 152D. In this example, windows 152Aand 152C have similar widths, but the widths of windows 152B and 152Dvary. More specifically, servo windows 152A, 152C and 152E define awidth (W₁), servo window 152B defines a width (W₂), servo window 152Ddefines a width (W₃), and servo window 152F defines a width (W₄).Because the widths of servo windows 152B, 152D, 152F are differentrelative to one another, a servo controller can distinguish signalsassociated with the different tracks 154A-154E. Accordingly, the needfor track identification marks, e.g. that conventionally cross thecenterlines, can be eliminated.

Moreover, in medium 150, each pair of servo windows that defines a giventrack 154 vary relative to one another. In other words, widths of servowindows 152A and 152B, which define track 154A are different relative toone another, widths of servo windows 152B and 152C, which define track154B are different relative to one another, widths of servo windows 152Cand 152D, which define track 154C, are different relative to oneanother, widths of servo windows 152D and 152E, which define track 154D,are different relative to one another, and widths of servo windows 152Eand 152F, which define track 154E are different relative to one another.As described above with reference to FIG. 13, varying the widths ofservo windows that are positioned above and below a given centerline ofa given track can provide inherent synchronization information withinthe servo windows. Accordingly, the need for additional synchronizationmarks that cross the centerlines can be eliminated.

In the embodiment illustrated in FIG. 15, second servo window 152B andfourth servo window 152D have varying widths. Alternatively, first servowindow 152A and third servo window 152C could be defined to have thehave varying widths to achieve track identification and inherentsynchronization information in the pattern of servo windows.

FIG. 16 is a depiction of the servo band portion of medium 160. Inaddition, FIG. 16 illustrates a magnetic head 162 relative to medium 160along centerline 165A of a first servo tracks of medium 160. In otherwords, head 162 passes over medium 160, or alternatively medium 160passes under head 162. FIG. 17 illustrates the corresponding readoutsignal 170 of magnetic head 160 as it passes relative to medium 160along centerline 165A as depicted in FIG. 16.

When head 162 passes relative to region 167 that is completely recorded,readout signal 170 assumes its 100% maximum value A₁. However, when head162 passes partially over one of servo windows 168A or 169A alongcenterline 165, signal 170 assumes value A₂, which is approximately 50%of the maximum value A₁. In other words, when head 162 is positionedprecisely along centerline 165 at one of servo windows 168A or 169A,one-half of head 162 detects the signal in region 167 and the other halfof head 162 passes over a non-recorded servo window 168A or 169A.

Readout signal 170 also provides a measure of widths W₁ and W₂, whichcorrespond to the widths of servo windows 168 and 169, respectively.Because widths W₁ and W₂ are different from each other, a controllerassociated with head 162 can analyze readout signal 170 and determinewhether an occurrence of amplitude A₂ corresponded to servo window 168Aabove centerline 165 or servo window 169A below centerline 165.Accordingly, the need for conventional synchronization marks in theservo pattern on medium 160 can be eliminated.

Moreover, the width of servo window 168B is different than the width ofservo window 168A, and the width of servo window 168C is different thanthe widths of either of servo windows 168A or 168B. Thus, the readoutsignals associated with different tracks (identified by centerlines165A-165E) can be distinguished by variance in width W₁. In particular,the readout signals for first track corresponding to centerline 165Awould be different than the readout signals for either of third or fifthtracks, corresponding to centerlines 165C and 165E respectively, becauseof variance in width W₁. A servo controller associated with head 162 cananalyze such variance in the readout signal in order to facilitate trackidentification without the need for conventional track identificationmarks that span across one or more centerlines 165.

FIG. 18 is another depiction of the servo band portion of medium 160. Inaddition, FIG. 16 illustrates a magnetic head 162 passing relative tomedium 160 along a line 185, which is slightly below the centerline 165Acorresponding to the first servo track (FIG. 16) of medium 160. FIG. 19illustrates the corresponding readout signal 190 of magnetic head 162 asit passes relative to medium 160 along line 185 as depicted in FIG. 18.

When head 162 passes relative to region 167 that is completely recorded,readout signal 190 assumes its 100% maximum value A₁. However, when head162 passes partially over one of servo windows 168A or 169A along line185, signal 190 assumes different values. In particular, when head 162passes partially over servo window 168A along line 185, signal 190assumes value A′₃, which is less than A₁, but larger than 50% of A₁.When head 162 passes partially over servo window 169A along line 185,signal 190 assumes value A′₂, which is less than 50% of A₁.

Readout signal 190 also provides a measure of widths W₁ and W₂, whichcorrespond to the widths of servo windows 168A and 169A, respectively.Moreover, relative to centerline 165C or centerline 165E (labeled inFIG. 16), the readout signal would also reflect variance in width W₁,which can facilitate track identification as described herein.

Because widths W₁ and W₂ are different from each other, a controllerassociated with head 162 can analyze readout signal 190 and determinethat A′₂ is associated with a window below the centerline and A′₃ isassociated with a window above the centerline. Thus, the controller candetermine that head 162 is off-track and needs to be moved laterallyupward. Again, synchronization marks are not needed because differingwidths of windows 168A and 169A allow for self-synchronization.Moreover, variance in width W₁ in the readout signal can allow thecontroller to determine which servo track is being read by head 162 at aparticular time.

FIG. 20 is another depiction of the servo band portion of medium 160. Inaddition, FIG. 20 illustrates a magnetic head 162 passing relative tomedium 160 along a line 205, which is slightly above centerline 165A ofthe first servo track (FIG. 16). FIG. 21 illustrates the correspondingreadout signal 210 of magnetic head 162 as it passes relative to medium160 along line 205 as depicted in FIG. 20.

When head 162 passes relative to region 167 that is completely recorded,readout signal 210 assumes its 100% maximum value A₁. However, when head162 passes partially over one of servo windows 168A or 168B along line205, signal 210 assumes different values. In particular, when head 162passes partially over one of servo window 168A along line 205, signal210 assumes value A″₃, which is less than 50% of A₁. When head 162passes partially over one of servo window 168B along line 205, signal210 assumes value A″₂, which is less than A₁, but larger than 50% of A₁.Therefore, a controller of head 162 can determine whether to move head162 up or down with respect to the centerline, in response to suchoff-track amplitude measurements based on the amplitude measurement andthe corresponding width measurement associated with the amplitude. Suchself-synchronization is highly desirable because it eliminates the needfor additional synchronization marks, as well as manufacturing effortsassociated with formation of additional synchronization marks.

Readout signal 210 also provides a measure of widths W₁ and W₂, whichcorrespond to the widths of servo windows 168A and 168B, respectively.Because widths W₁ and W₂ are different from each other, a controllerassociated with head 162 can analyze readout signal 210 and determinethat A″₂ is associated with a window below the centerline and A″₃ isassociated with a window above the centerline. Thus, the controller candetermine that head 162 is off-track and needs to be moved laterallydownward. Again, synchronization marks are not needed because differingwidths of windows 168A and 168B allows for self-synchronization.

Moreover, relative to centerline 165C or centerline 165E (labeled inFIG. 16), the readout signal would also reflect variance in width W₁,which can facilitate track identification as described herein. Inparticular, width W₁ would be reduced when head 162 moved over a line inclose proximity to centerline 165C relative to width W₁ depicted inFIGS. 17, 19 and 21. Similarly, width W₁ would be further reduced whenhead 162 moved over a line in close proximity to centerline 165E.Accordingly, measurement of width W₁ may provide the ability to identifytrack locations without the need for conventional track identificationmarks, e.g., that cross one or more centerlines.

FIG. 22 is a block diagram of a system 220 comprising a data storagemedium 221A in the form of magnetic tape, a servo read head 223, a dataread/write head 224, and a servo subsystem controller 225A to controlthe positioning of heads 223, 224 relative to medium 221A. In somecases, multiple servo read heads similar to head 223 may be used inorder to read differing patterns in adjacent servo bands, as describedabove, and thereby facilitate track identification. FIG. 22, however,relates to an embodiment making use of a single servo head 223, e.g.,for reading a servo pattern similar to that illustrated in FIGS. 14, 15or 16. In other words, medium 221A may correspond to medium 140 of FIG.14, medium 150 of FIG. 15, medium 160 of FIG. 16, or the like. On theother hand, if servo patterns similar to those illustrated in FIGS. 4,5A-5C, 9, 10 or 13 are used, two or more servo heads on a servo devicewould simultaneously read different tracks of the different servo bands.FIG. 23 is described below and provides more details of an embodimentfor reading servo patterns similar to those illustrated in FIGS. 4,5A-5C, 9, 10 or 13. In that case, all of the output signals would besent to the controller for decoding and collective analysis as describedabove. In particular, collective signals of a track in a first servoband and a corresponding track in a second servo band can be decoded tofacilitate track identification.

Referring to FIG. 22, medium 221A comprises magnetic tape spooled ontospools 211 and 212. In particular, medium 221A feeds from spool 211 tospool 212, passing in close proximity to servo head 223 and read/writehead 224 for magnetic recording and/or readout. For example, medium 221Amay contact heads 223, 224. Medium 221A generally corresponds to medium140, 150 or medium 160 described above, and includes servo windowsarranged to facilitate track identification and possiblysynchronization.

Servo head 223 detects servo signal amplitudes in medium 221 andprovides the detected signal to servo subsystem controller 225A. Signalconditioning components 215, such as amplifiers, pre-amplifiers,filters, or the like, condition the detected signal and provide thedetected signal to envelope detector 216, which generates an envelopesignal based on the detected and conditioned signal. The envelope signalis filtered by one or more filters 217, e.g., in order to round thecorners of square waves in the envelope signal. Differentiator 218generates pulses corresponding to the edges in the envelope signal. Thepulses provide information regarding the measured amplitude of thesignal and the distance between pulses provide information regarding thewidths of the servo windows. Also, offsets between pulses of differentwindows may be used for track identification.

Amplitude discriminator 227 differentiates signal from noise bycomparing the input signal to some fixed or variable threshold, such as50% of the average peak amplitude, and time discriminator 226 measuresthe timing between pulses in order to estimate the widths of the servowindows. Track identification (ID) unit 222A identifies the track beingread based on widths of servo windows or offsets between servo windowsof the track being read and provides such information to a drivecontroller. Position error signal (PES) generator 228 receives inputfrom amplitude discriminator 227 and time discriminator 226 andgenerates position error signals based on the measured amplitudes andwidths, and provides the position error signals to compensator 229.

Compensator 229 generates signals to adjust the lateral positioning ofheads 223, 224 relative to medium 221A in order to achieve on-trackpositioning of heads 223, 224 relative to medium 221A. Actuator 231applies the signals of compensator 229 in order to control movement ofheads 223, 224 relative to medium 221A. In this manner, servo subsystemcontroller 225A uses servo patterns on medium 221A, identifies the trackbeing read, and provides feedback control of positioning of heads 223,224 relative to medium 221A based on the detected servo patterns.

System 230 of FIG. 23 operates very similarly to system 220 of FIG. 22and includes many similar components that operate as outlined above.System 230, however, would be used if servo patterns similar to thoseillustrated in FIGS. 4, 5A-5C, 9, 10 or 13 are read. In other words,medium 221B may include servo patterns similar to those illustrated inFIGS. 4, 5A-5C, 9, 10 or 13. In that case, two or more servo heads 223A,223B are disposed on a servo device for simultaneous reading ofdifferent tracks of the different servo bands. The output signals of thedifferent servo heads 223A, 223B would be sent to servo subsystemcontroller 225B for decoding similar to that described above withreference to FIG. 22. However, the collective outputs of both servoheads 223A and 223B would also be analyzed, e.g., by a track ID unit222B. The track ID unit 222B, for example, would discern trackidentification based on offsets between servo windows of the differentbands of medium 221B, as described herein.

For both of FIGS. 22 and 23, the output of track ID unit 222A or 222B isprovided to a drive controller, which is the master controller forsystem 220 or 230. For example, the drive controller generally controlsdata communication between either of systems 220 or 230 and a hostcomputer, and also controls such things as tape and motor speeds and theaddressing of data on the medium. Accordingly, the drive controller mayuse the track identification information discerned by track ID unit 222Aor 222B in order to address the data to specific tracks and keep trackof such addressing in order to enable retrieval of the data.

By way of example, further details of the operation of a servo systemwill now be provided with reference to system 220 of FIG. 22. It isunderstood that system 230 of FIG. 23 would operate in a similar manner,duplicating the processing of signals from the two different servo heads223A, 223B, and then using offsets between the signals of differentbands for track identification purposes.

FIG. 24 illustrates a readout signal 240, that may be generated whenservo head 233 passes relative to a servo track substantially alongcenterline 165A (FIG. 16). Based on signal 240, envelope detector 216generates envelope signal 242. Then, following filtering by filter 217,differentiator 218 generates pulses 245A-245H (collectively pulses 245)as shown in FIG. 25, corresponding to the edges in envelope signal 242.

Amplitude discriminator 227 differentiates signal from noise bycomparing the input signal to some fixed or variable threshold, such as50% of the average peak amplitude, and time discriminator 226 measuresthe timing (T₁ and T₂) between pulses in order to estimate the widths ofservo windows 168A, 169A (FIG. 16). Track ID unit 222A identifies thetrack being read based on widths estimated by timing (T₁ and T₂), oroffsets between the pulses corresponding to different servo windows.

Position error signal (PES) generator 228 generates position errorsignals based on the measured amplitudes and widths, and provides theposition error signals to compensator 229. Compensator 229 uses theposition error signals to generate adjustment signals for actuator 231,which adjusts lateral positioning of heads 223, 224 relative to medium221A in order to achieve on-track positioning of heads 223, 224 relativeto medium 221A. In this case, actuator 231 does not adjust the lateralpositioning of heads 223, 224 because signal 240, signal 242, andcorresponding pulses 245 indicate that positioning is exactly on track(or at least within the minimum position error measurement tolerance ofthe system).

FIG. 26 illustrates a readout signal 260, that may be generated whenservo head 223 passes relative to a first track below centerline 165A(FIG. 16). Based on signal 260, envelope detector 216 generates envelopesignal 262. Then, following filtering by filter 217, differentiator 228generates pulses 265A-265H (collectively pulses 265) as shown in FIG.27, corresponding to the edges in envelope signal 262.

Amplitude discriminator 227 differentiates signal from noise bycomparing the input signal to some fixed or variable threshold, such as50% of the average peak amplitude, and time discriminator 226 measuresthe timing (T₁ and T₂) between pulses in order to estimate the widths ofservo windows 168A, 169A (FIG. 16) on medium 221A. Track ID unit 222Aidentifies the track being read based on widths estimated by timing (T₁and T₂), or offsets between the pulses corresponding to different servowindows.

Position error signal (PES) generator 228 generates position errorsignals based on the measured amplitudes, and provides the positionerror signals to compensator 229. Compensator 229 uses the positionerror signals to generate adjustment signals for actuator 231, whichadjusts lateral positioning of heads 223, 224 relative to medium 221A inorder to achieve on-track positioning of heads 223, 224 relative tomedium 221A.

In this case, actuator 231 causes heads 223, 224 to move upward becausesignal 260, signal 262, and corresponding pulses 265 indicate thatpositioning of heads 223, 224 is below centerline 165A (FIG. 16). Servosubsystem controller 225B is programmed to recognize, for every givenservo track, which of the larger and smaller servo windows resides aboveor below the centerline for that given track. For this reason,differently sized servo windows enable self synchronization, without theneed for additional synchronization marks in the servo pattern. Also,differently sized servo windows may facilitate track identification.

FIG. 28 illustrates a readout signal 280, that may be generated whenservo head 223 passes slightly above centerline 165A (FIG. 16). Based onsignal 280, envelope detector 226 generates envelope signal 282. Then,following filtering by filter 217, differentiator 218 generates pulses285A-285H (collectively pulses 285) as shown in FIG. 29, correspondingto the edges in envelope signal 282.

Amplitude discriminator 227 differentiates signal from noise bycomparing the input signal to some fixed or variable threshold, such as50% of the average peak amplitude, and time discriminator 226 measuresthe timing (T₁ and T₂) between pulses in order to estimate the widths ofservo windows 168A, 169A (FIG. 16) on medium 221A. Track ID unit 222Aidentifies the track being read based on widths estimated by timing (T₁and T₂), or offsets between the pulses corresponding to different servowindows.

Position error signal (PES) generator 228 generates position errorsignals based on the measured amplitudes, and provides the positionerror signals to compensator 229. Compensator 229 uses the positionerror signals to generate adjustment signals for actuator 231, whichadjusts lateral positioning of heads 223, 224 relative to medium 221A inorder to achieve on-track positioning of heads 223, 224 relative tomedium 221A. In this case, actuator 231 causes heads 223, 224 to movelaterally downward because signal 280, signal 282, and correspondingpulses 285 indicate that positioning of heads 223, 224 is belowcenterline 165A (FIG. 16). Again, servo subsystem controller 225A isprogrammed to recognize for every given track, which of the larger andsmaller servo windows resides above or below the centerline for thatgiven track, thereby allowing for self synchronization without the needfor additional synchronization marks in the servo pattern. Also,differently sized servo windows may facilitate track identification.

FIG. 30 illustrates an exemplary servo writing device 300 forpre-recording servo patterns on medium 301 as described herein. Medium301 comprises magnetic tape spooled onto spools 308 and 309 and can berecorded with servo patterns using servo writing device 300. Servowriting device 300 includes a servo write head 302, a servo erase head303, and a controller 305 to control the magnetic fields applied byheads 302, 303. Medium 301 feeds from spool 308 to spool 379, passing inclose proximity to heads 302, 303. For example, medium 301 may contactheads 302, 303 during recording.

Heads 302, 303 comprise electromagnetic elements that generate magneticfields. Controller 305 causes servo write head 302 to write a periodicpattern substantially over the full servo band associated with medium301. Then, controller 305 causes servo erase head 303 to selectivelyerase servo windows above and below centerlines of the various trackswithin the servo band. In accordance with the invention, the patterns ofservo windows are arranged to facilitate inherent track identificationand possibly synchronization without the need for conventionalsynchronization marks or track identification marks that span acrosscenterlines of the tracks. In some cases, different arrangements ofservo patterns are created for different servo bands. Offsets betweenservo windows of one track are different than offsets between servowindows of another track. In some cases, widths of the servo windows mayvary. In these ways, the patterns of servo windows can be arranged tofacilitate inherent track identification and possibly inherentsynchronization.

FIG. 31 is a cross-sectional conceptual view of an exemplary servo writehead 302 and a servo erase head 303 that form a servo writing device300. A magnetic shield 317 may be positioned between servo write head302 and servo erase head 303 in order to eliminate electrical ormagnetic interaction between the heads. Again, heads 302, 303 compriseelectromagnetic elements to generate magnetic fields. In particular,controller 305 (FIG. 30) applies electrical signals to heads 302, 303via coils 311, 312 in order to cause heads 302 to generate magneticfields across gaps 314, 315. For example, a periodic electrical signalmay be applied to head 302 via coil 311 in order to generate anoscillating magnetic field across gap(s) 314. Gap(s) 314 may be formeddirectly in an electromagnetic element to define head 302. Gap(s) 314may be relatively wide in the cross-tape direction such that themagnetic signal can be recorded over a full surface of the servo bands.

Controller 305 also applies an electrical signal to head 303 via coil312 in order to generate a magnetic field across gap(s) 315. Inparticular, a direct current electrical signal may be applied to head303, or alternatively, an alternating signal of substantially differentfrequency than that applied to head 302 may be applied to head 303. Ineither case, gaps 315 are arranged to define the servo pattern. Inparticular, head 303 may comprise an electromagnetic element 307 and amagnetic layer 308 formed over electromagnetic element 307. The magneticlayer 308 may be formed or etched to define a pattern of gaps, that inturn define the servo pattern. For example, magnetic layer 308 maycomprise a magnetically permeable layer that is deposited overelectromagnetic element 307 via masking techniques to define patterns ofgaps. Alternatively, magnetic layer 308 may comprise a magneticallypermeable layer that is deposited over electromagnetic element 307 andthen etched to define patterns of gaps. Also, magnetic layer 308 may bepreformed to define the gaps and then adhered to electromagnetic element307 to define head 303.

FIG. 32 is a top view of an exemplary servo writing device 320comprising a servo write head 322 and a servo erase head 323 separatedby a magnetic shield 327. Servo writing device 320 illustrated in FIG.32 may correspond to servo writing device 300 illustrated in FIGS. 30and 31. Servo write head 322 and servo erase head 323 are configured tosimultaneously record different servo patterns corresponding todifferent servo bands on a medium. In particular, heads 322 and 323 maybe used to create servo patterns similar to those of bands 41A and 41Bof FIG. 4. Servo write head 322 includes two relatively wide gaps 327Aand 327B. Servo erase head 323 includes sets of gaps 328, 329, e.g.,formed in a magnetic layer over an electromagnetic element. If desiredadditional wide gaps may be formed on servo write head 322 andadditional sets of gaps may be formed on servo erase head 323 forsimultaneous creation of servo patterns of additional servo bands.

As illustrated in FIG. 32, servo erase head 323 includes a first set ofgaps 328 formed over the electromagnetic element to define a firstmagnetic field pattern corresponding to a first servo band on a magneticmedium, and a second set of gaps 329 formed over the electromagneticelement to define a second magnetic field pattern corresponding to asecond servo band on the magnetic medium. First set of gaps 328 isarranged differently than the second set of gaps 329. In this example,first set of gaps 328 is arranged in a checkerboard-like configurationand second set of gaps 329 is arranged in a stepped configuration.

In operation, a generally continuous magnetic signal generated by servowrite head 322 at gaps 327A, 327B to record magnetic signals over servobands of a medium as the medium passes relative to heads 322, 323. Theservo controller applies electrical pulses to servo erase head 323 togenerate timed bursts of magnetic signals at sets of gaps 328, 329 asthe medium passes relative to heads 322, 323. With the medium movingrelative to heads 322, 323, the timed bursts of magnetic signals at setsof gaps 328, 329 create sets of servo windows similar to thoseillustrated in bands 41A and 41B of FIG. 4.

FIG. 33 is an alternative top view of a servo erase head 333 includes afirst set of gaps 338 formed over the electromagnetic element to definea first magnetic field pattern corresponding to a first servo band on amagnetic medium, and a second set of gaps 339 formed over theelectromagnetic element to define a second magnetic field patterncorresponding to a second servo band on the magnetic medium. First setof gaps 338 is arranged differently than the second set of gaps 339. Inthis example, first set of gaps 338 is arranged in an upward steppedconfiguration and second set of gaps 339 is arranged in a downwardstepped configuration. Servo erase head 333 may form part of a servodevice that also includes a servo write head. Such a servo writingdevice may be used to create sets of servo windows similar to thoseillustrated in bands 91A and 91B of FIG. 9. For each set of gaps 338,339, centerlines of tracks for the servo band are defined between eachstep in the stepped configuration of the gaps.

FIG. 34 is another alternative top view of a servo erase head 343 inaccordance with an embodiment of the invention. Servo erase head 343 mayform part of a servo device that also includes a servo write head. Sucha servo writing device may be used to create sets of servo windowssimilar to those illustrated in FIG. 14. Servo erase head 343 includesan electromagnetic element (not shown) to generate a magnetic field. Aset of gaps 345 is formed over the electromagnetic element to define amagnetic field pattern corresponding to a servo band on a magneticmedium. If desired, each of the gaps may include oval shaped terminatorson the ends of the gaps, which can improve the quality of the magneticfields which permeate from the gaps. Such terminators on the gaps may beused in gaps of any of the heads described herein.

The set of gaps 345 is arranged to define a first subset of gaps 347positioned above and below a location corresponding to a firstcenterline of the servo band and corresponding to a first servo track, asecond subset of gaps 348 positioned above and below a locationcorresponding to a another centerline (the third centerline) andcorresponding to a the third servo track of the servo band, and a thirdsubset of gaps 349 positioned above and below a location correspondingto yet another centerline of the servo band (the fifth centerline) andcorresponding to the fifth servo track of the servo band. The firstsubset of gaps 347, the second subset of gaps 348, and the third subsetof gaps 349 are unique relative to one another. In particular, thedistance between gaps in the first subset 347, second subset 348 andthird subset 349 differ such that offsets between servo window createdby gaps 345 will differ for the different servo tracks.

FIG. 35 is another alternative top view of a servo erase head 353 inaccordance with an embodiment of the invention. Servo erase head 353 mayform part of a servo device that also includes a servo write head. Sucha servo writing device may be used to create sets of servo windows onmagnetic tape as describe herein. Servo erase head 353 includes anelectromagnetic element (not shown) to generate a magnetic field. Setsof gaps 355A, 355B are formed over the electromagnetic element to definemagnetic field pattern corresponding to two servo bands on a magneticmedium. The arrangements of sets of gaps 355A, 355B differ relative toone another.

The sets of gaps 355A and 355B are arranged to define subsets of gapspositioned above and below locations corresponding first centerlines ofthe servo bands. For example subset of gaps 359A, 359B and 359Ccorrespond to a first servo track of a first servo band. In accordancewith the invention, some of the created servo windows will have largerwidths than other servo windows. In particular, individual gaps ofdifferent subsets may be displaced from one another in a manner thatdefines the widths of created servo windows. Gaps 359B and 359Ccollectively create one servo window that is wider than a servo windowcreated by gap 359A. The different sets of gaps 355A and 355B may alsobe formed on servo erase heads without the other set in order to createa single servo band on a medium. As with any servo erase head embodimentdescribed herein, the number of sets of gaps (and thus number of servobands being created) is subject to a wide variety of implementations.For example, a write head may include any number of gaps to record servoband signals, and any number of sets of erase gaps may be included todefine erased servo windows.

FIG. 36 is another alternative top view of a servo erase head 363 inaccordance with an embodiment of the invention. Servo erase head 363 mayform part of a servo device that also includes a servo write head. Sucha servo writing device may be used to create sets of servo windowssimilar to those illustrated in FIG. 15. Servo erase head 363 includesan electromagnetic element (not shown) to generate a magnetic field. Aset of gaps 365 is formed over the electromagnetic element to define amagnetic field pattern corresponding to a servo band on a magneticmedium.

The set of gaps 365 is arranged to define a first subset of gaps 367positioned above and below a location corresponding to a firstcenterline of the servo band and corresponding to a first servo track, asecond subset of gaps 368 positioned above and below a locationcorresponding to a third centerline and corresponding to a third servotrack of the servo band, and a third subset of gaps 369 positioned aboveand below a location corresponding to a fifth centerline of the servoband and corresponding to a fifth servo track of the servo band. Thefirst subset of gaps 367, the second subset of gaps 348, and the thirdsubset of gaps 369 are unique relative to one another. In particular,the distance between gaps in the first subset 367, second subset 368 andthird subset 369 differ such that widths of at least some of the servowindows created by gaps 345 will differ for the different servo tracks.In particular, individual gaps of different subsets may be displacedfrom one another in a manner that defines the widths of created servowindows.

FIG. 37 illustrates a servo erase head 373 similar to servo head 353(FIG. 35) passing over a medium 371 to erase servo windows in medium371. As medium 371 passes relative to head 373, a timed pulse ofelectrical current is delivered to head 373. The timed pulse createsmagnetic fields to define the servo windows. The timed pulse has aduration that, in conjunction with motion of medium 371, defines a servowindow width for each gap of servo erase head 373. In some cases, two ormore gaps collectively define servo windows having widths larger thanthose servo windows defined by a single gap. For example, gaps 375 worktogether to create a servo window 377 that is wider than windows createdby a single gap. Medium 371 may comprise magnetic tape that typicallypasses relative to head 373 at a rate between approximately 0.1 meterper second and 30 meters per second, although any tape speed could beused. The duration of the time pulse of electrical current delivered tohead 373 may be dependent on the tape speed and may be in the range of0.5 to 300 micro-seconds, although the invention is not necessarilylimited in that respect. In accordance with the invention, gaps 373 canbe defined in a layer formed over an electromagnetic element asdescribed herein. Accordingly, separate current sources are not requiredfor each individual gaps, but rather a common current source creates thepattern of magnetic fields via gaps 375. The same exemplary tape speedand pulse duration times mentioned above may also apply to the otherheads described herein, e.g., the heads described with reference toFIGS. 30-36 having other arrangements of gaps formed on a layer over anelectromagnetic element.

FIG. 38 is a depiction of medium 380 including a plurality of servobands and data bands between the servo bands. In accordance with theinvention, any number of servo bands may be created. The servo writingdevices described above may be modified to include any number of sets ofgaps needed to simultaneously create the servo patterns of the differentbands. Any of the servo bands of medium 380 may include variousarrangements of servo windows described herein. In some cases, thearrangements of different bands are different in order to facilitatetrack identification. For example, the arrangements of servo windows ofdifferent bands may alternate, with every other servo band having asimilar pattern, but adjacent servo bands defining different patterns.

A number of embodiments of the invention have been described. Forexample, numerous different servo patterns comprising amplitude-basedservo windows have been described. Moreover, servo head configurationsand techniques for recording such patterns have also been described.Nevertheless, various modifications may be made without departing fromthe scope of the invention. For example, the servo windows could bewritten with signals, rather than erased, as outlined herein. Inaddition, the invention could be used with other media that makes use ofpre-recorded servo patterns, such as magnetic disks, holographic media,or the like. Accordingly, other embodiments are within the scope of thefollowing claims.

1. A servo writing device comprising: a first electromagnetic element togenerate a magnetic field to record signal on a magnetic medium; asecond electromagnetic element to selectively erase the signal; a layerformed over the second electromagnetic element to define a set of gaps;and a controller to control the second electromagnetic element such thata magnetic field pattern is generated from the set of gaps for definedperiods of time as the magnetic medium passes relative to the servowriting device such that amplitude-based servo windows are created onthe medium by the set of gaps.
 2. The servo writing device of claim 1,wherein the defined period of time is between approximately 0.5 and 300micro-seconds and the medium passes relative to the servo writing deviceat a speed between approximately 0.1 and 30 meters per second.
 3. Aservo writing device comprising: an electromagnetic element to generatea magnetic field; a layer formed over the electromagnetic element todefine a first set of gaps that define a first magnetic field patterncorresponding to a first servo band on a magnetic medium and a secondset of gaps that define a second magnetic field pattern corresponding toa second servo band on the magnetic medium.
 4. The servo writing deviceof claim 3, wherein the first set of gaps is arranged differently thanthe second set of gaps.
 5. The servo writing device of claim 3, whereinthe first set of gaps is arranged in a checkerboard-like configurationand the second set of gaps is arranged in a stepped configuration. 6.The servo writing device of claim 4, wherein the first set of gaps isarranged in an upward stepped configuration and the second set of gapsis arranged in a downward stepped configuration.
 7. The servo writingdevice of claim 3, further comprising a controller to control theelectromagnet such that the first and second magnetic field patterns aregenerated for defined periods of time as a magnetic medium passesrelative to the servo writing device such that servo windows are createdon the medium by the first and second sets of gaps.
 8. The servo writingdevice of claim 7, wherein the gaps in one of the first and second setsare arranged such that some of the servo windows have larger widths thanother servo windows.
 9. The servo writing device of claim 3, furthercomprising a magnetically permeable layer formed over theelectromagnetic element, wherein the first and second sets of gaps areetched in the magnetically permeable layer.
 10. A servo writing devicecomprising: a first electromagnetic element to generate a magnetic fieldto record signal on a magnetic medium; a second electromagnetic elementto selectively erase the signal; a layer formed over the secondelectromagnetic element to define a first set of gaps that define afirst pattern corresponding to a first servo band on a magnetic mediumand a second set of gaps that define a second pattern corresponding to asecond servo band on the magnetic medium.
 11. The servo writing deviceof claim 10, wherein the second electromagnetic element selectivelyerases the signal by generating a different magnetic field whichselectively records a different signal on the magnetic medium via thefirst and second sets of gaps.
 12. The servo writing device of claim 10,wherein the second electromagnetic element selectively erase the signalby generating a direct current (DC) magnetic field to selectively recorda different signal via the first and second sets of gaps.
 13. The servowriting device of claim 10, wherein the first set of gaps is arrangeddifferently than the second set of gaps.
 14. The servo writing device ofclaim 13, wherein the first set of gaps is arranged in acheckerboard-like configuration and the second set of gaps is arrangedin a stepped configuration.
 15. The servo writing device of claim 13,wherein the first set of gaps is arranged in an upward steppedconfiguration and the second set of gaps is arranged in a downwardstepped configuration.
 16. The servo writing device of claim 10, furthercomprising a controller to control the electromagnet such that the firstand second magnetic field patterns are generated for defined periods oftime as a magnetic medium passes relative to the head such thatamplitude-based servo windows are created on the medium by the first andsecond sets of gaps.
 17. The servo writing device of claim 16, whereinthe gaps in the first or second set are arranged such that some of theamplitude-based servo windows have larger widths than otheramplitude-based servo windows.
 18. The servo writing device of claim 10,wherein the layer formed over the second electromagnetic elementcomprises a magnetically permeable layer, wherein the first and secondsets of gaps are etched in the magnetically permeable layer.
 19. Theservo writing device of claim 10, wherein the first electromagneticelement defines a plurality of relatively wide gaps to record aplurality of signals on the magnetic medium in a plurality of servobands.
 20. A servo writing device comprising: an electromagnetic elementto generate a magnetic field; and a layer formed over theelectromagnetic element to define a set of gaps that define a magneticfield pattern corresponding to a servo band on a magnetic medium,wherein the set of gaps is arranged in an stepped configuration in whichcenterlines of tracks for the servo band are defined between each stepin the stepped configuration of the gaps.
 21. The servo writing deviceof claim 20, further comprising a controller to control theelectromagnetic element such that the magnetic field pattern isgenerated for defined periods of time as the magnetic medium passesrelative to the servo writing device and such that servo windows arecreated on the medium by the set of gaps.
 22. The servo writing deviceof claim 20, wherein the stepped configuration comprises an upwardstepped configuration in the direction perpendicular to the servotracks.
 23. The servo writing device of claim 20, wherein the steppedconfiguration comprises a downward stepped configuration in thedirection perpendicular to the servo tracks.
 24. A servo writing devicecomprising: an electromagnetic element to generate a magnetic field; anda layer formed over the electromagnetic element to define a set of gapsthat define a magnetic field pattern corresponding to a servo band on amagnetic medium, wherein the set of gaps is arranged to define a firstsubset of gaps positioned above and below a location corresponding to afirst centerline of the servo band and corresponding to a first servotrack, a second subset of gaps positioned above and below a locationcorresponding to a third centerline and corresponding to a third servotrack of the servo band, and a third subset of gaps positioned above andbelow a location corresponding to a fifth centerline of the servo bandand corresponding to a fifth servo track of the servo band, wherein thefirst subset of gaps, the second subset of gaps, and the third subset ofgaps are unique relative to one another.
 25. The servo writing device ofclaim 24, further comprising a controller to control the electromagneticelement such that the magnetic field pattern is generated for definedperiods of time as the magnetic medium passes relative to the servowriting device such that amplitude-based servo windows are created onthe medium by the set of gaps.
 26. The servo writing device of claim 24,wherein a distance between the gaps in the first subset that are aboveand below the first centerline is different than a distance between thegaps in the third subset that are above and below the fifth centerline.27. The data storage medium of claim 24, wherein the gaps in the firstsubset include a different number of gaps below the centerline thanabove the centerline such that a width of an amplitude-based servowindow created by the gaps in the first subset above the centerline isdifferent than a width of an amplitude-based servo window created by thegaps in the first subset below the centerline.